A surface-mount type piezoelectric device is one in which a piezoelectric vibrating piece is mounted on an insulating package base made of a material such as alumina ceramic. A package lid is then sealed to the package base to enclose the piezoelectric vibrating piece. During manufacture of such a piezoelectric device, a layer of sealing material, such as a polymeric resin or low-melting-point glass (LMPG), is formed on a sealing surface formed either on the package base or on the package lid. Using resin for forming a sealing layer can cause problems, such as parametric fluctuations, because polymeric resins tend to release gas under the elevated temperatures required for curing the resin. Even if the sealing layer is formed of LMPG, entrapped bubbles in the LMPG can release gas, causing adverse effects to the piezoelectric vibrating piece inside the package.
Japan Patent Publication No. JP 2005-026974A discloses a method for releasing unwanted gas from inside the package. On the entire edge-surface of the package base made of a non-conductive material, a first layer of LMPG is applied for temporary hardening. Then, a layer of a second LMPG is applied over the first layer and temporarily hardened. The second LMPG is not applied to pre-designated regions of the surface of the second LMPG, particularly regions connected to inside the package. These regions provide escape routes for release of gas from the package before completion of package sealing.
The method disclosed in JP '974 disadvantageously requires two or more applications of LMPG and respective temporary hardening steps, which is process-intensive. Also, if the second LMPG has low viscosity, then the second LMPG tends to spread, before it has temporarily hardened, to regions where LMPG is not wanted. Replacing the low-viscosity LMPG with a higher-viscosity LMPG poses difficulties in sealing, due to the problem of the LMPG not flowing to surrounding regions. Furthermore, the manufacturing methods discussed in JP '974 require applying LMPG on each individual piezoelectric device, which is unsuitable for mass-production.
Therefore, there is a need for methods for manufacturing piezoelectric devices, as disclosed herein, that do not result in entrapment of unwanted gas or water vapor inside the package containing the piezoelectric device. There is also a need for piezoelectric vibrating devices that do not contain unwanted gas or water vapor.